Common hard disk drives are storage devices comprising disks whose data-carrying surfaces are coated with a magnetic layer. Typically, the disks are positioned atop one another on a disk stack (platters) and rotate around an axis, or spindle. To store data, each disk surface is organized in a plurality of circular, concentric tracks. Groups of concentric tracks placed atop each other in the disk stack are called cylinders. Read/write heads, each containing a read element and a write element, are mounted on an actuator arm and are moved over the spinning disks to a selected track, where the data transfer occurs. The actuator arm is controlled by a hard disk controller, an internal logic responsible for read and write access.
Each track on a disk surface is divided into sections, or segments, known as physical sectors (which must not be confused with the geometrical circle sector). A physical sector, also referred to as a data block or sector data, typically stores a data unit of 512 bytes or 4 KB of user data.
A disk surface may be divided into zones. Zones are regions wherein each track comprises the same number of physical sectors. From the outside inward, the number of physical sectors per track may decrease from zone to zone. This approach is known as zone bit recording.
The storage capacity of a hard disk drive can be increased, inter alia, by reducing the track pitch (i.e., track width) of the concentric tracks on the disk surfaces. This requires a decrease in the size of the read and write elements. However, without new storage technologies, a reduction in the size of the write elements is questionable, as the magnetic field that can be generated is otherwise too small to adequately magnetize the individual bits on the disk surface. A known solution is the shingled magnetic recording (SMR) methodology, by which a write element writes data tracks in an overlapping fashion.
With SMR, overlapping data tracks are grouped into bands, which are separated by inter-band gaps, also known as “guard bands,” “guard regions,” “guard tracks,” or “guard segments.” Typically, to change the contents of a first track in an already populated band, it is necessary to read out and buffer all subsequent tracks of the band because after updating the data on that first track, rewriting the buffered data up to the next guard region is unavoidable as the wide write element will inevitably overwrite the data of each subsequent track. Due to the sequential and overlapping structure of SMR, even a small change to the contents stored in a band can result in a significant increase in the amount of data that must be read and written, thus leading to significant delays. Such a process is referred to as “read-modify-write” or “write amplification.” Further information pertaining to shingled magnetic recording can be found in U.S. Pat. No. 8,223,458 B2, U.S. Pat. No. 8,432,633 B2, and U.S. Pat. No. 8,179,627 B2, as well as in patent applications US2012/0082019 A1 and US2013/0148225 A1.
A computer, or host, accessing a hard disk drive may use logical block addresses (LBAs) in commands to read and write sector data without regard for the actual locations of the physical sectors on the disc surfaces. By means of a hard disk controller the logical block addresses (LBAs) can be mapped to physical block addresses (PBAs) representing the physical locations of sector data. Different mapping techniques for an indirect LBA-to-PBA read and write access are known in the prior art. In some embodiments LBA-to-PBA mapping does not change often. In other embodiments the LBA-to-PBA mapping may change with every write operation, the physical sectors being assigned dynamically.
Sector data read from a physical sector may be subjected to a forward error correction. For this purpose, additional error-correcting codes may be included in the data stored on the physical sector. The hard disk controller may monitor whether physical sectors are poorly legible, e.g., by means of the information derived from the forward error correction. If a physical sector is no longer legible, the hard disk controller may report a CRC error.
A physical sector that is poorly legible or no longer legible is sometimes called a “bad sector” and will be referred to herein as unreliable or defective sector. Hard disk drives may autonomously “repair” defective sectors during regular operation by means of defect management. A defective sector may be replaced by a spare sector from a spare sector area that has been reserved for this purpose. The reference to the spare sector may be stored in a G-list (grown defects list). This is referred to as G-list remapping. Defect management processes may be logged by a monitoring system such as S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology).
Moreover, when producing a hard disk drive, the manufacturer may recognize unreliable or defective sectors on the disk surfaces that can be mapped out by means of a P-list (primary defects list) so that the hard disk drive skips these unreliable or defective sections. This is referred to as “sector slipping” or just “slipping.”
In the cases of both G-list remapping (grown defects) and P-list remapping (primary defects), mapping out the defects may be achieved by changing the association between the logical block addresses (LBAs) and the physical block addresses (PBAs) of the affected physical sectors. To the computer or host, the logical blocks still appear to be error-free. Remapped sector data may, nevertheless, affect the access time in some embodiments and, as soon as all spare sectors from the spare sector areas are in use, it is time to replace the hard disk drive.
U.S. Pat. No. 7,408,731 B2, entitled “Track allocation method of disk drive,” describes a disk surface having at least two types of track widths, i.e., an area with overlapping tracks (“shingled tracks,” ST) and an area with conventional, non-overlapping tracks (“tiled track,” TT). Storage space that is missing due to a defective track is regained by adding additional overlapping and thus space-saving tracks to the “shingled tracks” area (ST). Defective tracks are simply skipped while writing the bands.